This invention relates to an improved process for oligomerizing ethylene using zinc-promoted, nickel-based catalyst compositions and, more particularly, to a process for oligomerizing ethylene to a product rich in linear butenes using a catalyst composition made by incorporating a major amount of (1) a HAMS-1B crystalline borosilicate molecular sieve composited in an inorganic support or (2) an amorphous, high-silica-content, silica-alumina support with minor amounts of a nickel compound and a zinc compound and calcining the result to form a composite containing supported zinc and nickel oxides.
Supported nickel oxide catalysts, such as NiO on silica-alumina supports, have been known as active catalysts for the dimerization of ethylene to butenes since the 1950s. See, for example, U.S. Pat. Nos. 2,381,198; 2,581,228; 2,606,940; 3,341,720; and 4,628,139. Since these catalysts produce significant quantities of C.sub.5 + olefins along with butenes, principally hexenes, octenes, decenes and other higher oligomers of ethylene, they are more appropriately termed ethylene oligomerization or dimerization/oligomerization catalysts.
Interest in making linear C.sub.4 olefins from ethylene has increased in recent years as a way to utilize ethylene when it is in long supply and also as a way to make 2-butene for alkylation purposes. In addition, since isomerization within the C.sub.4 olefin group is well-known, making a linear C.sub.4 olefin in good yield is a way to make isobutylene which is useful as an intermediate for t-butyl alcohol and t-butyl methyl ether.
Thus, it is of interest to find catalysts of increased level of activity which are able to convert ethylene more selectively into butenes at higher feed rates, lower reaction pressures or with more dilute ethylene streams and simultaneously minimize production of the higher oligomers.
Now it has been found that by combining a small amount of a zinc compound with a small amount of a nickel compound on a high-silica-content silica-alumina or a supported HAMS-1B crystalline borosilicate molecular sieve, catalyst compositions can be made which show considerably enhanced oligomerization activity while still being reasonably selective in forming linear C.sub.4 olefins. This synergistic effect found in mixed nickel-zinc catalyst compositions can be employed to make compositions commercially useful under less severe process conditions.